It is known that when a vinyl aromatic hydrocarbon and a conjugated diene are subjected to block copolymerization by living anionic polymerization in an organic solvent using an alkyl lithium as an initiator, the structure of the copolymer can be diversified by a method of e.g. changing the weight ratio of the vinyl aromatic hydrocarbon to the conjugated diene or changing the manner of addition, whereby block copolymers having various physical properties can be obtained: In general, a block copolymer is a polymer having excellent impact resistance and transparency. If the content of the conjugated diene in the block copolymer is large, the copolymer tends to be a thermoplastic elastomer. On the other hand, if the content of the vinyl aromatic hydrocarbon is large, the copolymer tends to show characteristics as thermoplastics. To utilize such excellent characteristics, various production methods have been disclosed, for example, in JP-B-36-19286 and JP-B-48-4106.
The block copolymer comprising a vinyl aromatic hydrocarbon and a conjugated diene produced by these methods has a high transparency and favorable moldability, and therefore as disclosed in JP-B-63-49702, a heat shrinkable film having such characteristics that it shrinks in a short time at a temperature of at least the shrinkage initiation temperature, while it keeps the original shape at room temperature, can be obtained by using the resin as a raw material. This heat shrinkable film is produced by carrying out a processing operation wherein the resin extruded into a plate shape or a tube shape at a temperature of at least the softening point of the resin is oriented in a uniaxial or a biaxial direction to obtain a film, which is directly cooled to room temperature. By carrying out the series of processing operation including orienting, an individual molecule constituting the film is fixed as aligned in a specific direction, whereby the internal stress remains, and the heat shrinkable film is a film utilizing the shrinking phenomenon caused by that the above-described residual stress is relieved by re-heating.
Further, in addition to these excellent properties, it is excellent in miscibility with various vinyl aromatic hydrocarbon polymers, and it is thereby used for reinforcement, which is disclosed, for example, in JP-B-45-19388, JP-B-47-43618 and JP-B-51-27701. However, such block copolymers and compositions are relatively transparent, have favorable impact resistance and their oriented films show heat shrinkability, whereby they are favorably employed as various heat shrinkable packaging materials and labeling materials for beverage bottles on which tradenames and the like are printed. Particularly with respect to shrinkable labeling materials, as a result of increase in demand for plastic bottle beverages in recent years, line speed in a labeling step has been increased year by year, and reduction of time required for labeling and shrinkage has been attempted, and accordingly a film which requires a short time for heating, i.e. a film which shrinks at a low temperature, has been desired. However, a conventional film for shrink labels has a high heat shrinkage initiation temperature, and shrinkage at a relatively high temperature is required so as to obtain favorable shrinkage finish.
Further, if the molecular structure or the composition formulation is set so that the film starts to shrink at a low temperature merely by a conventional method, the heat shrinkable film shrinks during storage (so-called spontaneous shrinkage), thus causing printing slippage or labeling failure. As mentioned above, with respect to a film obtained by a conventional technique, no material having a balance among low temperature shrinkability and spontaneous shrinkage resistance, and outer appearance after shrinkage, has been obtained, and further, surface roughening may form on the film surface, thus impairing the outer appearance, in some cases. In JP-A-59-221348 and Japanese Patent No. 3,026,497, it has been studied to blend a styrene-acrylate type resin to obtain a multilayer film, from the viewpoint of reduction of spontaneous shrinkage and improvement in low temperature shrinkability of a styrene-butadiene block copolymer. However, it is not satisfactory from the viewpoint of e.g. needs for low temperature shrinkability in recent years and odor, and further, width of conditions at the time of orienting the film tends to be narrow, such being unsatisfactory.
It is an object of the present invention to provide a copolymer and its composition, capable of forming a film which satisfies needs such as low temperature shrinkability in recent years, spontaneous shrinkage resistance and reduction of odor, and which is excellent in outer appearance, for application as e.g. a heat shrinkable film, and a heat shrinkable film and a heat shrinkable multilayer film made of said copolymer or its composition.